Display device

ABSTRACT

A display device which includes a display panel and a protection member above the display panel. The protection member may include an adhesive layer above the display panel, a first protection layer above the adhesive layer, and a second protection layer between the first protection layer and the adhesive layer. The first protection layer may have a first thickness and a first modulus, and the second protection layer may have a second thickness less than the first thickness and a second modulus less than the first modulus. The minimum value of the second thickness may be 20 μm, and the maximum value of the second thickness may satisfy Equation (1) below. The display device, which includes the protection member including the second protection layer satisfying Equation (1), may improve impact resistance and maintain reliability. 
         T 2 max =(0.13× T 1−8.25)×ln  M 2+0.68× T 1−5.1  (1)

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to and the benefit of KoreanPatent Application No. 10-2021-0099811, filed on Jul. 29, 2021, theentire content of which is hereby incorporated by reference.

BACKGROUND

Aspects of some embodiments of the present disclosure herein relate to adisplay device.

Display devices provide users with information by displaying variousimages on a display screen. Generally, display devices display theinformation within an allocated screen. Recently, flexible displaydevices including a foldable and/or flexible display panel have beendeveloped. Such a flexible display device may be folded, rolled, orbent, without damaging the integrity or functionality of the displaydevice, as opposed to rigid display devices, which would generally bedamaged by such motions or actions. The flexible display devicedeformable into various shapes may enable various screen sizes, andthus, the user convenience may be improved.

A protection member may be arranged or formed on the uppermost portionof the display device. The protection member may have impact resistanceand reliability characteristics to protect the overall foldable flexibledisplay device from damage.

The above information disclosed in this Background section is only forenhancement of understanding of the background and therefore theinformation discussed in this Background section does not necessarilyconstitute prior art.

SUMMARY

Aspects of some embodiments of the present disclosure herein relate to adisplay device, and for example, to a display device which includes aprotection member arranged on a display panel.

Aspects of some embodiments of the present disclosure include a displaydevice in which impact resistance and reliability are maintained.

According to some embodiments, a display device includes: a displaypanel which is foldable about at least one folding axis; and aprotection member above the display panel, wherein the protection memberincludes: an adhesive layer located above the display panel; a firstprotection layer which is located above the adhesive layer, and has afirst thickness and a first-first modulus at room temperature; and asecond protection layer which is located between the adhesive layer andthe first protection layer, and has a second thickness less than thefirst thickness and a second-first modulus less than the first-firstmodulus at room temperature, wherein the minimum value of the secondthickness is about 20 μm, and the maximum value of the second thicknesssatisfies Equation (1) below:

T2_(max)=(0.13×T1−8.25)×ln M2+0.68×T1−5.1  (1)

wherein in Equation (1), T2 _(max) is the maximum value of the secondthickness, T1 is the first thickness, and M2 is a second-second modulusof the second protection layer at about 60° C.

According to some embodiments, moduli of the adhesive layer may be lessthan moduli of the first protection layer and moduli of the secondprotection layer at room temperature and about 60° C.

According to some embodiments, the second-first modulus of the secondprotection layer may be about 10 MPa to about 500 MPa, and thesecond-second modulus may be about 36 MPa to about 400 MPa.

According to some embodiments, the first-first modulus of the firstprotection layer may be about 4 GPa to about 5 GPa, and the firstprotection layer may have a first-second modulus at about 60° C., andthe first-second modulus may be about 4.2 GPa to about 4.4 GPa.

According to some embodiments, a modulus of the adhesive layer at about60° C. may be about 10 KPa to about 40 KPa, and a modulus at roomtemperature may be about 20 KPa to about 50 KPa.

According to some embodiments, a thickness of the adhesive layer may beabout 35 μm to about 80 μm.

According to some embodiments, the first protection layer may include atleast one of polyethylene terephthalate (PET), polybutyleneterephthalate (PBT), polyethylene naphthalate (PEN), polycarbonate (PC),poly methyl methacrylate (PMMA), polystyrene (PS), polyvinyl chloride(PVC), polyethersulfone (PES), polyethylene (PE), polypropylene (PP),polyamide (PA), modified-polyphenylene oxide (m-PPO), polyoxymethylene(POM), polyarylsulfones (PSU), polyphenylene sulfide (PPS), polyimide(PI), polyethylenimine (PEI), polyether ether ketone (PEEK),polyamide-imide (PAI), or polyarylate (PAR).

According to some embodiments, the second protection layer may includeat least one of thermoplastic polyurethane (TPU), thermoplasticpolyester elastomer (TPEE), thermoplastic vulcanizate (TPV),thermoplastic olefin (TPO), thermoplastic starch (TPS), ethylenepropylene rubber (EPDM), or polyether block amide (PEBA).

According to some embodiments, the first protection layer may includepolyethylene terephthalate, and the second protection layer includesthermoplastic polyurethane.

According to some embodiments, the protection member may further includea functional layer located above the first protection layer, and thefunctional layer may include at least one of an anti-fingerprint layeror a hard coating layer.

According to some embodiments, a modulus of the functional layer at roomtemperature may be about 2 GPa to about 4 GPa.

According to some embodiments, a thickness of the functional layer maybe about 5 μm to about 10 μm.

According to some embodiments, the protection member may further includean auxiliary adhesive layer located between the first protection layerand the second protection layer, and wherein moduli of the auxiliaryadhesive layer are less than moduli of the first protection layer andmoduli of the second protection layer at room temperature and about 60°C.

According to some embodiments, the moduli of the auxiliary adhesivelayer may be greater than or equal to moduli of the adhesive layer atroom temperature and about 60° C.

According to some embodiments, the first thickness of the firstprotection layer may be about 65 μm to about 100 μm.

According to some embodiments, the second thickness of the secondprotection layer may be about 20 μm to about 95 μm.

According to some embodiments, a thickness of the protection member maybe about 200 μm to about 250 μm.

According to some embodiments of the inventive concept, a display deviceincludes: a display panel which is foldable about at least one foldingaxis; and a protection member located above the display panel, whereinthe protection member includes: a first protection layer which islocated above the display panel, and has a first thickness and afirst-first modulus at room temperature; a second protection layer whichis located below the first protection layer, and has a second thicknessless than the first thickness and a second-first modulus less than thefirst-first modulus at room temperature; and an adhesive layer which islocated between the display panel and the second protection layer, andhas a third-first modulus less than the second-first modulus at roomtemperature, wherein the minimum value of the second thickness is about20 μm, and the maximum value of the second thickness satisfies Equation(1) below:

T2_(max)=(0.13×T1−8.25)×ln M2+0.68×T1−5.1  (1)

wherein in Equation (1), T2_(max) is the maximum value of the secondthickness, T1 is the first thickness, and M2 is a second-second modulusof the second protection layer at about 60° C.

According to some embodiments, a thickness of the protection member maybe about 200 μm to about 250 μm, the first thickness of the firstprotection layer may be about 65 μm to about 100 μm, the secondthickness of the second protection layer may be about 95 μm or less, anda thickness of the adhesive layer may be about 35 μm to about 80 μm.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings are included to provide a furtherunderstanding of embodiments according to the inventive concept, and areincorporated in and constitute a part of this specification. Thedrawings illustrate embodiments of the inventive concept and, togetherwith the description, serve to explain characteristics of someembodiments of the inventive concept. In the drawings:

FIG. 1A is a perspective view showing a display device according to someembodiments;

FIG. 1B is a perspective view showing a display device according to someembodiments;

FIG. 2 is a cross-sectional view taken along the line I-I′ of FIG. 1A;

FIG. 3 is a cross-sectional view showing a display device according tosome embodiments;

FIG. 4 is a cross-sectional view showing a display device according tosome embodiments;

FIG. 5A is a graph showing buckling indexes according to thicknesses ofa second protection layer according to some embodiments; and

FIG. 5B is a graph showing buckling indexes according to thicknesses ofa second protection layer according to some embodiments.

DETAILED DESCRIPTION

In this specification, when an element (or a region, a layer, a portion,or the like) is referred to as “being on”, “being connected to”, or“being coupled to” another element, it may be directly locatedon/connected/coupled to another element, or an intervening third elementmay also be arranged therebetween.

Like numbers refer to like elements throughout. Also, in the drawings,the thicknesses, ratios, and dimensions of the elements are exaggeratedfor effective description of the technical contents. “And/or” includesone or more combinations which may be defined by the associatedelements.

Although the terms first, second, etc. may be used to describe variouselements, these elements should not be limited by these terms. Theseterms are only used to distinguish one element from another element. Forexample, a first element may be referred to as a second element, andsimilarly, a second element may also be referred to as a first elementwithout departing from the scope of the present disclosure. The singularforms include the plural forms as well, unless the context clearlyindicates otherwise.

Also, terms of “below”, “on lower side”, “above”, “on upper side”, orthe like may be used to describe the relationships of the elementsillustrated in the drawings. These terms have relative concepts and aredescribed on the basis of the directions indicated in the drawings.

It will be understood that the term “includes” or “comprises”, when usedin this specification, specifies the presence of stated features,integers, steps, operations, elements, components, or a combinationthereof, but does not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components, orcombinations thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which the present disclosure belongs.Also, terms, such as those defined in commonly used dictionaries, shouldbe interpreted as having a meaning that is consistent with their meaningin the context of the relevant art and should not be interpreted in anidealized or overly formal sense unless expressly so defined herein.

Hereinafter, aspects of some embodiments of the inventive concept willbe described in more detail with reference to the drawings. FIGS. 1A and1B are perspective views showing a display device DD according to someembodiments.

A display device DD may be a device to be activated in response to anelectrical signal. For example, the display device DD may be a personaldigital terminal, a tablet PC, a vehicle navigation unit, a gameconsole, or a wearable device, but the embodiments of the inventiveconcept are not limited thereto. In FIGS. 1A and 1B, the display deviceDD is illustrate as an example as a mobile electronic device.

The display device DD according to some embodiments may be flexible.“Being flexible” represents the characteristics of being bendable, andmay encompass all structures from a completely foldable structure to astructure bendable to the order of several nanometers. For example, thedisplay device DD may be a foldable display device. Also, the displaydevice DD may be rigid.

The thickness direction of the display device DD may be parallel to athird direction axis DR3 that is a normal direction of the plane definedby the first direction axis DR1 and the second direction axis DR2. Thedirections indicated as the first to third direction axes DR1, DR2, andDR3 illustrated in this specification may have a relative concept andthus may be changed to other directions. Also, the directions indicatedas the first to third direction axes DR1, DR2, and DR3 may be referredto as first to third directions, and the same reference symbols may beused therefor. In this specification, the first direction axis DR1 isperpendicular to the second direction axis DR2, and the third directionaxis DR3 may be a normal direction of the plane defined by the firstdirection axis DR1 and the second direction axis DR2.

The display device DD may include a first display surface FS which isparallel to the plane defined by the first direction axis DR1 and thesecond direction axis DR2 crossing the first direction axis DR1. Thedisplay device DD may display an image IM through the first displaysurface FS. The display device DD may display the image IM, in the thirddirection axis DR3, through the first display surface FS parallel toeach of the first direction axis DR1 and the second direction axis DR2.

The display device DD may include the first display surface FS and asecond display surface RS. The first display surface FS may include afirst active area F-AA and a first peripheral area F-NAA. A module areaEMA may be provided in the first active area F-AA. The display device DDmay include an electronic module arranged so as to correspond to themodule area EMA. For example, the electronic module may include one of acamera, a light detection sensor, or a heat detection sensor.

The display device DD may display the image IM through the first activearea F-AA. Also, various types of external inputs may be sensed in thefirst active area F-AA. The first peripheral area F-NAA may be adjacentto the first active area F-AA. The first peripheral area F-NAA maysurround the first active area F-AA. The shape of the first active areaF-AA may be substantially defined by the first peripheral area F-NAA.Unlike the configuration illustrated in FIG. 1A, the first peripheralarea F-NAA may be arranged adjacent to only one side of the first activearea F-AA, or the first peripheral area F-NAA may be omitted. Thedisplay device DD may include active areas having various shapes and isnot limited to one embodiment.

The second display surface RS may be defined as a surface which faces atleast a portion of the first display surface FS. The second displaysurface RS may be defined as a portion of the rear surface of thedisplay device DD.

The display device DD may include a folding area FA1 and non-foldingareas NFA1 and NFA2. The display device DD may include a firstnon-folding area NFA1 and a second non-folding area NFA2 with onefolding area FA1 arranged therebetween. However, this is merely anexample, and the number of folding areas and the number of non-foldingareas are not limited thereto. For example, embodiments according to thepresent disclosure may include any suitable number of folding areas andnon-folding areas according to the design of the display device DD.

FIG. 1B is a perspective view showing an in-folding operation of thedisplay device DD of FIG. 1A. The display device DD may be folded abouta folding axis FX1 parallel to the second direction axis DR2. FIG. 1Billustrates one folding axis FX1, but the number of folding axes in thedisplay device DD according to some embodiments is not limited thereto.

When the display device DD is in-folded, the first non-folding area NFA1and the second non-folding area NFA2 may face each other. When thedisplay device DD is in-folded, the first display surface FS may not beexposed to a user, but the second display surface RS may be exposed tothe user. On the other hand, the display device DD may be out-folded sothat the first display surface FS is exposed, but the second displaysurface RS is not exposed.

FIG. 2 is a cross-sectional view taken along the line I-I′ of FIG. 1A.The display device DD according to some embodiments may include adisplay panel DP and a protection member PM arranged above the displaypanel DP. The protection member PM will be described later in moredetail with reference to FIGS. 3 and 4 .

Also, the display device DD may include a window adhesive layer PSA-WMand a window WM which are arranged between the display panel DP and theprotection member PM. An auxiliary functional layer SFL and a supportmember SP may be arranged below the display panel DP.

The support member SP may include a first support member MM1 and asecond support member MM2. The first support member MM1 may be spacedapart from the second support member MM2 in parallel to a direction inwhich the first direction axis DR1 extends. The support member SP mayoverlap the non-folding areas NFA1 and NFA2, but may not overlap thefolding area FA1. The first support member MM1 may overlap the firstnon-folding area NFA1, but may not overlap the folding area FA1. Thesecond support member MM2 may overlap the second non-folding area NFA2,but may not overlap the folding area FA1. However, the embodiments arenot limited thereto, and at least a portion of each of the first supportmember MM1 and the second support member MM2 may overlap the foldingarea FA1.

The support member SP may include a single layer or multi layers. Thesupport member SP may include at least one of a heat dissipating member,a shielding member, or an insulating member. Each of the first supportmember MM1 and the second support member MM2 may be made of a metalalloy. For example, each of the first support member MM1 and the secondsupport member MM2 may include stainless steel, aluminum, copper, or analloy thereof. However, this is merely an example, and materialscontained in the first support member MM1 and the second support memberMM2 are not limited thereto.

The auxiliary functional layer SFL may be arranged on the support memberSP. The auxiliary functional layer SFL may include a single layer ormulti layers. For example, the auxiliary functional layer SFL mayinclude at least one of a cushion layer, a barrier film, or a protectionfilm. The auxiliary functional layer SFL may include a cushion layermade of foam or sponge. The auxiliary functional layer SFL may includethe barrier film that prevents or reduces deformation of the displaypanel DP. The auxiliary functional layer SFL may include a coloredpolyimide film as a protection film. However, this is merely an example,and materials contained in the auxiliary functional layer SFL andfunctions of the auxiliary functional layer SFL are not limited thereto.

The display panel DP may be arranged above the auxiliary functionallayer SFL. The display panel DP may be folded about the folding axis FX1(see FIG. 1 ). The display panel DP may include a display element layer.For example, the display element layer may include an organic lightemitting element, a quantum-dot light emitting element, a liquid crystalelement layer, or the like. However, this is merely an example, and theembodiments are not limited thereto.

The window WM may be optically transparent. The window WM may include asingle layer or multi layers. For example, the window WM may includeglass or polyimide (PI) resin. However, this is merely an example, andmaterials contained in the window WM are not limited thereto.

The window adhesive layer PSA-WM may be arranged between the window WMand the display panel DP. The window WM may be coupled to the displaypanel DP by the window adhesive layer PSA-WM. The window adhesive layerPSA-WM may include a pressure sensitive adhesive film (PSA), anoptically clear adhesive film (OCA), or an optically clear resin (OCR).In the display device DD according to some embodiments, the windowadhesive layer PSA-WM may be omitted.

FIG. 3 is a cross-sectional view showing a protection member PMaccording to some embodiments. FIG. 3 shows the protection member PM ofFIG. 2 in more detail.

The protection member PM according to some embodiments may include anadhesive layer PSA-PM arranged above the display panel DP, a firstprotection layer PL1 arranged above the adhesive layer PSA-PM, and asecond protection layer PL2 arranged between the adhesive layer PSA-PMand the first protection layer PL1. For example, the protection memberPM may be arranged above the window WM, and the adhesive layer PSA-PM ofthe protection member PM may be arranged above the protection member PM.The protection member PM may be coupled to the window WM by the adhesivelayer PSA-PM.

According to some embodiments, the second protection layer PL2 may havea thickness less than that of the first protection layer PL1, and mayhave a modulus less than that of the first protection layer PL1. Also,the second protection layer PL2 may have a thickness greater than thatof the adhesive layer PSA-PM, and may have a modulus greater than thatof the adhesive layer PSA-PM. The protection member PM, which includesthe second protection layer PL2 having the modulus less than that of thefirst protection layer PL1 and greater than that of the adhesive layerPSA-PM and having the thickness less than that of the first protectionlayer PL1 and greater than that of the adhesive layer PSA-PM, mayexhibit characteristics in which the impact resistance and reliabilityare maintained. In the specification, the modulus is an elastic modulus.

A thickness T0 of the protection member PM may be about 200 μm to about250 μm. When the thickness of the protection member is less than about200 μm, the protection member and/or components (for example, thewindow, etc.) arranged below the protection member are damaged byexternal impact. The protection member does not protect the componentsarranged below when the external impact occurs, and the protectionmember and/or the components arranged below the protection member aredamaged. Also, when the thickness of the protection member is greaterthan about 250 μm, the protection member is damaged when the displaydevice is folded and unfolded. The protection member having thethickness greater than about 250 μm is not easily folded and unfolded,and the protection member may be peeled off from the display device orcracked. When the thickness of the protection member is greater thanabout 250 μm, the thickness of the display device including theprotection member increases. Thus, the display device DD, which includesthe protection member PM having the thickness T0 of about 200 μm toabout 250 μm, may have the improved impact resistance and maintain thereliability when repeatedly folded and unfolded.

The first protection layer PL1 may have a first thickness T1. The firstthickness T1 of the first protection layer PL1 may be about 65 μm toabout 100 μm. Among the components included in the protection member PM,the first protection layer PL1 may have the greatest thickness which isthe first thickness T1 . That is, as a proportion of the overallthickness T0 of the protection member PM, the first protection layer PL1may be thicker than any of the other layers of the protection member PM,according to some embodiments. The first thickness T1 of the firstprotection layer PL1 may be greater than the thicknesses of the secondprotection layer PL2 and the adhesive layer PSA-PM, which will bedescribed in more detail later. For example, the first thickness T1 ofthe first protection layer PL1 may be less than or equal to about 50% ofthe thickness T0 of the protection member PM. The thickness T0 of theprotection member PM may be about 200 μm, and the first thickness T1 ofthe first protection layer PL1 may be about 100 μm. However, this ismerely an example, and the ratio of the first thickness T1 of the firstprotection layer PL1 to the thickness T0 of the protection member PM isnot limited thereto.

When the thickness of the first protection layer is less than about 65μm, the first protection layer and/or components (for example, thesecond protection layer, and the window, etc.) arranged below the firstprotection layer are damaged. The first protection layer having thethickness less than about 65 μm is vulnerable to the impact, and thus,when the impact is applied to the first protection layer, the firstprotection layer and/or the components arranged below the firstprotection layer are damaged.

Also, the first protection layer having the thickness greater than about100 μm is damaged when the display device is folded and unfolded. Thefirst protection layer having the thickness greater than about 100 μm isnot easily folded and unfolded, and is peeled off from the displaydevice or cracked when folded and unfolded. According to someembodiments, the first protection layer PL1 having the thickness T1 ofabout 65 μm to about 100 μm may protect the components arranged belowthe first protection layer PL1 against external impact, and thereliability may be maintained even when the display device DD isrepeatedly folded and unfolded.

The first protection layer PL1 may have a first-first modulus at roomtemperature. The first-first modulus of the first protection layer PL1may be about 4GPa to about 5 GPa. For example, the first-first modulusof the first protection layer PL1 may be about 4.5 GPa to about 5 GPa.When the modulus of the first protection layer is less than about 4 GPaor the modulus of the first protection layer is greater than about 5GPa, the first protection layer is cracked or peeled off from thedisplay device when the display device is folded or unfolded. In theprotection member PM according to some embodiments, the first protectionlayer PL1 having the modulus of about 4 GPa to about 5 GPa at roomtemperature may maintain the impact resistance and reliability.

Also, the first protection layer PL1 may have a first-second modulus ofabout 4.2 GPa to about 4.4 GPa at about 60° C. The first protectionlayer PL1 having the first-second modulus of about 4.2 GPa to about 4.4GPa at about 60° C. may maintain the impact resistance and reliabilityeven under a relatively high temperature condition.

According to some embodiments, the second protection layer PL2 may havea thickness less than that of the first protection layer PL1. A secondthickness T2 of the second protection layer PL2 may be less than thefirst thickness T1 of the first protection layer PL1. The minimum valueof the second thickness T2 of the second protection layer PL2 may beabout 20 μm. The maximum value of the second thickness T2 of the secondprotection layer PL2 may satisfy Equation (1) below.

T2_(max)=(0.13×T1−8.25)×In M2+0.68×T1−5.1  (1)

In Equation (1), ‘T2 _(max)’ is the maximum value of the secondthickness T2 of the second protection layer PL2, and ‘T1’ is the firstthickness of the first protection layer PL1. ‘M2’ is a second-secondmodulus of the second protection layer PL2 at about 60° C. The secondprotection layer PL2 may have a second-first modulus at room temperatureand may have a second-second modulus at about 60° C.

In Equation (1), the maximum value T2 _(max) of the thickness of thesecond protection layer PL2 may be proportional to the first thicknessT1 of the first protection layer PL1. As the first thickness T1 of thefirst protection layer PL1 increases, the maximum value T2 _(max) of thethickness of the second protection layer PL2 may increase. Also, themaximum value T2 _(max) of the thickness of the second protection layerPL2 may be proportional to the second-second modulus of the secondprotection layer PL2 at about 60° C. As the second-second modulus of thesecond protection layer PL2 at about 60° C. has a higher value, themaximum value T2 _(max) of the thickness of the second protection layerPL2 may increase.

The second-second modulus of the second protection layer PL2 at about60° C. may be about 36 MPa to about 400 MPa. When the modulus of thesecond protection layer at about 60° C. is less than about 36 MPa or themodulus of the second protection layer at about 60° C. is greater thanabout 400 MPa, the second protection layer may be peeled off from thedisplay device or cracked when the display device is folded or unfolded.

At about 60° C. compared to room temperature, the peeling off (that is,buckling) of the protection member may be more serious. According tosome embodiments, the display device DD, which includes the protectionmember PM including the second protection layer PL2 having the modulusof about 36 MPa to about 400 MPa at about 60° C., may maintain theimpact resistance and reliability even under a relatively hightemperature condition.

According to some embodiments, the second thickness T2 of the secondprotection layer PL2 may be less than the first thickness T1 of thefirst protection layer PL1. The second thickness T2 of the secondprotection layer PL2 may be about 20 μm to about 95 μm. That is, themaximum value T2 _(max) of the second thickness T2 of the secondprotection layer PL2 may be about 95 μm.

For example, in Equation (1), when the second-second modulus M2 of thesecond protection layer PL2 at about 60° C. is about 36 MPa and thefirst thickness T1 of the first protection layer PL1 is about 65 μm, themaximum value T2 _(max) of the thickness of the second protection layerPL2 may be about 40 μm. The second thickness T2 of the second protectionlayer PL2, which is arranged below the first protection layer PL1 havingthe thickness of about 65 μm and has the second-second modulus of about36 MPa at about 60° C., may be about 20 μm to about 40 μm.

In Equation (1), when the second-second modulus M2 of the secondprotection layer PL2 at about 60° C. is about 36 MPa and the firstthickness T1 of the first protection layer PL1 is about 100 μm, themaximum value T2 _(max) of the thickness of the second protection layerPL2 may be about 80 μm. The second thickness T2 of the second protectionlayer PL2, which is arranged below the first protection layer PL1 havingthe thickness of about 100 μm and has the second-second modulus of about36 MPa at about 60° C., may be about 20 μmto about 80 μm.

In Equation (1), when the second-second modulus M2 of the secondprotection layer PL2 at about 60° C. is about 124 MPa and the firstthickness T1 of the first protection layer PL1 is about 65 μm, themaximum value T2 _(max) of the thickness of the second protection layerPL2 may be about 40 μm. The second thickness T2 of the second protectionlayer PL2, which is arranged below the first protection layer PL1 havingthe thickness of about 65 μm and has the second-second modulus of about124 MPa at about 60° C., may be about 20 μm to about 40 μm.

In Equation (1), when the second-second modulus M2 of the secondprotection layer PL2 at about 60° C. is about 124 MPa and the firstthickness T1 of the first protection layer PL1 is about 100 μm, themaximum value T2 _(max) of the thickness of the second protection layerPL2 may be about 86 μm. The second thickness T2 of the second protectionlayer PL2, which is arranged below the first protection layer PL1 havingthe thickness of about 100 μm and has the second-second modulus of about124 MPa at about 60° C., may be about 20 μm to about 86 μm.

In Equation (1), when the second-second modulus M2 of the secondprotection layer PL2 at about 60° C. is about 383 MPa and the firstthickness T1 of the first protection layer PL1 is about 65 μm, themaximum value T2 _(max) of the thickness of the second protection layerPL2 may be about 41 μm. The second thickness T2 of the second protectionlayer PL2, which is arranged below the first protection layer PL1 havingthe thickness of about 65 μm and has the second-second modulus of about383 MPa at about 60° C., may be about 20 μm to about 41 μm.

In Equation (1), when the second-second modulus M2 of the secondprotection layer PL2 at about 60° C. is about 383 MPa and the firstthickness T1 of the first protection layer PL1 is about 100 μm, themaximum value T2 _(max) of the thickness of the second protection layerPL2 may be about 92 μm. The second thickness T2 of the second protectionlayer PL2, which is arranged below the first protection layer PL1 havingthe thickness of about 100 μm and has the second-second modulus of about383 MPa at about 60° C., may be about 20 μm to about 92 μm. However,this is merely an example, and the first thickness T1 of the firstprotection layer PL1 and the second-second modulus of the secondprotection layer PL2 at about 60° C. are not limited thereto.

At room temperature, the second-first modulus of the second protectionlayer PL2 may be less than the first-first modulus of the firstprotection layer PL1. The second-first modulus may be about 10 MPa toabout 500 MPa. When the modulus of the second protection layer at roomtemperature is less than about 10 MPa or the modulus of the secondprotection layer at room temperature is greater than about 500 MPa, thesecond protection layer is not easily folded and unfolded, and thesecond protection layer is peeled off from the display device or crackedwhen repeatedly folded and unfolded. In the protection member PMaccording to some embodiments, the second protection layer PL2, whichhas the second-first modulus of about 10 MPa to about 500 MPa less thanthe first-first modulus of the first protection layer PL1, maycontribute to improving the impact resistance of the display device DDand maintaining the reliability.

The protection member PM, which includes the second protection layer PL2in which the maximum value T2 _(max) of the thickness satisfies Equation(1), may have a buckling index of about 0.5 or less. The buckling indexmay be defined as a ratio of a relative load to a critical force. Thecritical force is a minimal force which generates buckling when a forceis applied to press a column having a length (e.g., a set orpredetermined length) in one direction.

The critical force satisfies Equation (2) below, and the buckling indexsatisfies Equation (3) below, and this is the equation relating to Eulerbuckling theory.

$\begin{matrix}{F_{c} = \frac{\pi^{2}{EI}}{({kL})^{2}}} & (2)\end{matrix}$

In Equation (2), F_(c) is a critical force, E is a modulus of a column,and I is moment of inertia of the column. L is the overall length of thecolumn, and kL is a length in one direction corresponding to a portionof the column in which buckling occurs.

$\begin{matrix}{\frac{1}{BRI} = \frac{F}{F_{C}}} & (3)\end{matrix}$

In Equation (3), BRI is a buckling index, and F is the cross-sectionalarea of the column times a compressive force. As the buckling indexincreases, the possibility of the occurrence of buckling increases. Asthe buckling index of the protection member has a higher value, thepossibility of the occurrence of buckling of the protection memberincreases when repeatedly folded and unfolded. As the buckling index ofthe protection member has a higher value, the reliability of the displaydevice including the protection member deteriorates.

According to some embodiments, the protection member PM, which includesthe second protection layer PL2 in which the maximum value of thethickness satisfies Equation (1), may have a buckling index of about 0.5or less. The possibility of the occurrence of buckling in the protectionmember when repeatedly folded and unfolded increases in the protectionmember having a buckling index greater than 0.5, compared to aprotection member having a buckling index of about 0.5 or less. Theprotection member PM having a buckling index of about 0.5 or less is notpeeled off from the display device DD when repeatedly folded andunfolded, and may contribute to maintaining the reliability of thedisplay device DD. Thus, the display device DD, which includes theprotection member PM having a buckling index of about 0.5 or less, maymaintain the reliability when repeatedly folded and unfolded.

A thickness T3 of the adhesive layer PSA-PM may be less than the firstthickness T1 of the first protection layer PL1 and the second thicknessT2 of the second protection layer PL2. The thickness T3 of the adhesivelayer PSA-PM may be determined by taking into consideration a bucklingindex. Whenever the thickness T3 of the adhesive layer PSA-PM increasesby about 5 μm, the buckling index increases by about 0.01. Table 1 belowshows buckling indexes of the protection member PM according to thethicknesses of the adhesive layer PSA-PM.

In Table 1, each of protection members in Experimental example A and

Experimental example B includes an adhesive layer, a second protectionlayer arranged above the adhesive layer, and a first protection layerarranged above the second protection layer. In Experimental example Aand Experimental example B, the thicknesses of the first protectionlayers and the thicknesses of the second protection layers are equal toeach other, but thicknesses of the adhesive layers are different fromeach other. In the protection members of Experimental example A andExperimental example B, the thickness of the first protection layer isabout 100 μm, the thickness of the second protection layer is about 60μm, and the modulus of the second protection layer at about 60° C. isabout 124 MPa. In the protection members of Experimental example A andExperimental example B, the thickness of the first protection layercorresponds to the thickness range of about 65 μm to about 100 μm forthe first protection layer according to some embodiments. When themaximum value of the thickness of the second protection layer in each ofthe protection members of Experimental example A and Experimentalexample B is calculated by using Equation (1), the maximum value isabout 86 μm. That is, in the protection members of Experimental exampleA and Experimental example B, the thickness of the second protectionlayer corresponds to the thickness range of about 20 μm to about 80 μmfor the second protection layer according to Equation (1).

In Experimental example A and Experimental example B, the firstprotection layer includes polyethylene terephthalate (PET), the secondprotection layer includes thermoplastic polyurethane (TPU), and theadhesive layer includes a pressure sensitive adhesive film (PSA). InExperimental example A, the thickness of the adhesive layer is about 35μm. In Experimental example B, the thickness of the adhesive layer isabout 50 μm.

TABLE 1 Thickness of Buckling Classification adhesive layer indexExperimental 35 μm 0.39 example A Experimental 50 μm 0.42 example B

Referring to Table 1, it may be seen that the buckling index increasesby about 0.03 as the thickness of the adhesive layer increases by about15 μm. When a change in thickness of the adhesive layer is about 5 μm, achange in buckling index is about 0.01. That is, it may be seen that thebuckling index increases as the thickness of the adhesive layerincreases. Thus, the thickness of the second protection layer isdetermined from the thickness of the first protection layer by usingEquation (1), and the thickness of the adhesive layer may be adjusted bytaking into consideration the buckling index.

According to some embodiments, the thickness T3 of the adhesive layerPSA-PM may be about 35 μm to about 80 μm. When the thickness of theadhesive layer is less than about 35 μm, the coupling force between acomponent arranged above the adhesive layer and a component arrangedbelow the adhesive layer deteriorates because the adhesive layer is toothin. When the thickness of the adhesive layer is greater than about 80μm, the display device is not easily folded and unfolded, and thethickness of the display device increases. According to someembodiments, the protection member PM, which includes the adhesive layerPSA-PM of about 35 μm to about 80 μm, may contribute to maintaining thereliability of the display device DD. Also, the protection member PM,which includes the adhesive layer PSA-PM of about 35 μm to about 80 μm,may maintain a suitable thickness of the display device DD.

The adhesive layer PSA-PM may have a third-first modulus at roomtemperature and may have a third-second modulus at about 60° C. At roomtemperature, the third-first modulus of the adhesive layer PSA-PM may beless than the first-first modulus of the first protection layer PL1 andthe second-first modulus of the second protection layer PL2. Thethird-first modulus of the adhesive layer PSA-PM at room temperature maybe about 20 KPa to about 50 KPa. A protection member, which includes anadhesive layer having a modulus less than about 20 KPa or having amodulus greater than about 50 KPa at room temperature, is not easilyfolded and unfolded, and is peeled off from the display device orcracked when folded and unfolded. According to some embodiments, theprotection member PM, which includes the adhesive layer PSA-PM havingthe third-first modulus of about 20 KPa to about 50 KPa at roomtemperature, may contribute to maintaining the reliability of thedisplay device DD.

Also, third-second modulus of the adhesive layer PSA-PM at about 60° C.may be about 10 KPa to about 40 KPa. The adhesive layer PSA-PM havingthe third-second modulus of about 10 KPa to about 40 KPa at about 60° C.may maintain the impact resistance and reliability.

According to some embodiments, each of the first protection layer PL1and the second protection layer PL2 may include an optically transparentpolymer material. For example, the first protection layer PL1 mayinclude at least one of polyethylene terephthalate (PET), polybutyleneterephthalate (PBT), polyethylene naphthalate (PEN), polycarbonate (PC),poly methyl methacrylate (PMMA), polystyrene (PS), polyvinyl chloride(PVC), polyethersulfone (PES), polyethylene (PE), polypropylene (PP),polyimide (PA), modified-polyphenylene oxide (m-PPO), polyoxymethylene(POM), polyarylsulfones (PSU), polyphenylene sulfide (PPS), polyimide(P1), polyethylenimine (PEI), polyether ether ketone (PEEK),polyamide-imide (PAI), or polyarylate (PAR).

The second protection layer PL2 may include at least one ofthermoplastic polyurethane (TPU), thermoplastic polyester elastomer(TPEE), thermoplastic vulcanizate (TPV), thermoplastic olefin (TPO),thermoplastic starch (TPS), ethylene propylene rubber (EPDM), orpolyether block amide (PEBA).

For example, the first protection layer PL1 may include polyethyleneterephthalate, and the second protection layer PL2 may includethermoplastic polyurethane. However, materials included in the firstprotection layer PL1 and the second protection layer PL2 are not limitedthereto, and the second protection layer PL2 may include elastomerhaving a modulus less than that of the first protection layer PL1.

The second protection layer PL2 may be provided below the firstprotection layer PL1 through direct coating. One surface of the firstprotection layer PL1 may be directly coated with a material for formingthe second protection layer PL2. However, this is merely an example, anda method for forming the second protection layer PL2 is not limitedthereto.

According to some embodiments, the protection member PM may furtherinclude a functional layer FL. The functional layer FL may be arrangedabove the first protection layer PL1. For example, the functional layerFL may include at least one of a hard coating layer or ananti-fingerprint layer. The functional layer FL may include a singlelayer or multi layers. The functional layer FL made of a single layermay include both a hard coating function and an anti-fingerprintfunction. On the other hand, the functional layer FL made of multilayers may respectively include a hard coating layer and ananti-fingerprint layer.

A modulus of the functional layer FL may be equal to the first-firstmodulus of the first protection layer PL1 or may be less than thefirst-first modulus of the first protection layer PL1. The modulus ofthe functional layer FL at room temperature may be about 2 GPa to about4 GPa. A functional layer, which has a modulus less than about 2 GPa orhas a modulus greater than about 4 GPa, is not easily folded andunfolded. A protection member, which includes the functional layerhaving a modulus less than about 2 GPa or having a modulus greater thanabout 4 GPa, may be peeled off from the display device or cracked whenrepeatedly folded and unfolded. According to some embodiments, theprotection member PM, which includes the functional layer FL having themodulus of about 2 GPa to about 4 GPa, may contribute to maintaining thereliability of the display device DD.

A thickness T4 of the functional layer FL may be about 5 μm to about 10μm. When a protection member, which includes a functional layer having athickness less than about 5 μm, is subjected to external impact, thefunctional layer and/or a component (for example, the first protectionlayer) arranged below the functional layer are damaged. Also, aprotection member, which includes a functional layer having a thicknessgreater than about 10 μm, may be peeled off from the display device whenrepeatedly folded and unfolded. The protection member PM, which includesthe functional layer FL having the thickness T4 of about 5 μm to about10 μm, may improve the impact resistance. Also, the display device DDincluding the protection member PM may improve the impact resistance andmaintain the reliability.

FIG. 4 is a cross-sectional view showing a protection member PM-1according to some embodiments. Unlike FIG. 3 , the protection memberPM-1 of FIG. 4 may further include an auxiliary adhesive layer PSA-PL.The auxiliary adhesive layer PSA-PL may be provided between the firstprotection layer PL1 and the second protection layer PL2. The firstprotection layer PL1 may be coupled to the second protection layer PL2by the auxiliary adhesive layer PSA-PL. For example, the auxiliaryadhesive layer PSA-PL may include a pressure sensitive adhesive film(PSA), an optically clear adhesive film (OCA), or an optically clearresin (OCR).

According to some embodiments, moduli of the auxiliary adhesive layerPSA-PL at room temperature and about 60° C. may be less than the moduliof the first protection layer PL1 and the moduli of the secondprotection layer PL2.

The modulus of the auxiliary adhesive layer PSA-PL at room temperaturemay be less than the first-first modulus of the first protection layerPL1 and the second- first modulus of the second protection layer PL2.The modulus of the auxiliary adhesive layer PSA-PL at about 60° C. maybe less than the first-second modulus of the first protection layer PL1and the second-second modulus of the second protection layer PL2. Themoduli of the auxiliary adhesive layer PSA-PL at room temperature andabout 60° C. may be equal to or greater than the moduli of the adhesivelayer PSA-PM. The modulus of the auxiliary adhesive layer PSA-PL at roomtemperature may be equal to or greater than the third-first modulus ofthe adhesive layer PSA-PM. The modulus of the auxiliary adhesive layerPSA-PL about 60° C. may be equal to or greater than the third-secondmodulus of the adhesive layer PSA-PM. For example, the moduli of theauxiliary adhesive layer PSA-PL at room temperature and 60° C. may havesubstantially similar levels with the moduli of the adhesive layerPSA-PM.

A thickness T5 of the auxiliary adhesive layer PSA-PL may be less thanthe thickness T3 of the adhesive layer PSA-PM. For example, thethickness of the auxiliary adhesive layer PSA-PL may be about 10 μm toabout 35 μm. The protection member PM, which includes the auxiliaryadhesive layer PSA-PL having the thickness of about 10 μm to about 35μm, may contribute to maintaining the impact resistance and reliabilityof the display device DD. The auxiliary adhesive layer having athickness less than about 10 μm exhibits deteriorated coupling forcewith respect to components arranged below and above the auxiliaryadhesive layer. The auxiliary adhesive layer having a thickness greaterthan about 35 μm is not easily folded and unfolded, and the thickness ofthe display device increases.

The protection member according to the related art includes an adhesivelayer and one protection layer arranged above the adhesive layer, andthe one protection layer may be made of a single material. A protectionmember including one protection layer made of a single material isvulnerable to impact (or pressure), and the external impact is notalleviated or absorbed, but transmitted to a component (for example, awindow) arranged below the protection member. Accordingly, theprotection member and/or the component arranged below the protectionmember may be damaged, and the reliability of the display device may bedeteriorated.

Each of the protection members PM and PM-1 of embodiments includes theadhesive layer PSA-PM, the second protection layer PL2 arranged abovethe adhesive layer PSA-PM, and the first protection layer PL1 arrangedabove the second protection layer PL2, and the maximum value of thethickness of the second protection layer PL2 may satisfy Equation (1)described above. The second thickness T2 of the second protection layerPL2 may be less than the first thickness T1 of the first protectionlayer PL1 and greater than the thickness T3 of the adhesive layerPSA-PM, and the modulus of the second protection layer PL2 may be lessthan the modulus of the first protection layer PL1 and greater than themodulus of the adhesive layer PSA-PM. Each of the protection members PMand PM-1, which includes the first protection layer PL1, the secondprotection layer PL2, and the adhesive layer PSA-PM having differentmoduli and in which the modulus is reduced from an upper portion towarda lower portion, may exhibit the improved impact resistancecharacteristics.

When impacts are applied from the outside, rates at which the impactsare spread out in the first protection layer PL1, the second protectionlayer PL2, and the adhesive layer PSA-PM are different from each otherbecause the moduli of the first protection layer PL1, the secondprotection layer PL2, and the adhesive layer PSA-PM are different fromeach other. As the rates at which the impacts are spread out aredifferent from each other, levels of the impacts passing through theprotection members PM and PM-1 may be alleviated, or the impacts may beabsorbed by the protection members PM and PM-1. Thus, the displaydevices DD including the protection members PM and PM-1 according tosome embodiments may improve the impact resistance. Also, each of theprotection members PM and PM-1, which includes the second protectionlayer PL2 having the second thickness T2 less than the first thicknessT1 of the first protection layer PL1 and greater than the thickness T3of the adhesive layer PSA-PM, may maintain a suitable thickness of thedisplay device DD and contribute to maintaining the reliability of thedisplay device DD when folded and unfolded.

FIGS. 5A and 5B are graphs showing buckling indexes according to thethicknesses of the second protection layer. FIGS. 5A shows bucklingindexes when the thicknesses of second protection layers are about 20μm, about 40 μm, about 60 pm, about 80 μm, about 100 μm, and about 120μm in protection members of Experimental examples 1 to 3. FIGS. 5B showsbuckling indexes when the thicknesses of second protection layers areabout 20 μm, about 40 μm, about 60 μm, about 80 μm, about 100 μm, andabout 120 μm in protection members of Experimental examples 4 to 6.

Each of the second protection layers in the protection members ofExperimental examples 1 to 6 includes thermoplastic polyurethane (TPU),and each of first protection layers includes polyethylene terephthalate(PET) having the modulus of about 4 GPa at room temperature. Also, anadhesive layer having the thickness of about 35 μm may be provided ineach of Experimental examples 1 to 6.

A modulus, at room temperature, of each of the first protection layersin the protection members of Experimental examples 1 to 6 corresponds tothe range of about 4 GPa to about 5 GPa for the first-first modulus ofthe first protection layer at room temperature in some embodiments.Also, the thickness of each of the adhesive layers in the protectionmembers of Experimental examples 1 to 6 corresponds to the thicknessrange of about 35 μm to about 80 μm for the adhesive layer in someembodiments. The thicknesses of the first protection layers and themoduli, at about 60° C., of the second protection layers in theprotection members of Experimental examples 1 to 6 are shown in Table 2below.

TABLE 2 Thickness of first Modulus of second Classification protectionlayer protection layer, 60° C. Experimental  65 μm  36 MPa example 1Experimental 100 μm  36 MPa example 2 Experimental  65 μm 124 MPaexample 3 Experimental 100 μm 124 MPa example 4 Experimental  65 μm 383MPa example 5 Experimental 100 μm 383 MPa example 6

Referring to Table 2, the thickness of the first protection layer in theprotection members of Experimental examples 1, 3, and 5 is about 65 μm,and the thickness of the first protection layer in the protectionmembers of Experimental examples 2, 4, and 6 is about 100 μm. In theprotection members of Experimental examples 1 and 2, the modulus of thesecond protection layer at about 60° C. is about 36 MPa. In theprotection members of Experimental examples 3 and 4, the modulus of thesecond protection layer at about 60° C. is about 124 MPa. In theprotection members of Experimental examples 5 and 6, the modulus of thesecond protection layer at about 60° C. is about 383 MPa. In theprotection members of Experimental examples 1 to 6, the thickness of thefirst protection layer corresponds to the thickness range of about 65 μmto about 100 μm for the first protection layer according to someembodiments. Also, in the protection members of Experimental examples 1to 6, the modulus of the second protection layer at about 60° C.corresponds to the range of about 36 MPa to about 400 MPa for thesecond-second modulus of the second protection layer at about 60° C.according to some embodiments.

Table 3 below shows the buckling indexes according to the thicknesses ofthe second protection layers in the protection members of Experimentalexamples 1 to 6. That is, Table 3 shows the buckling indexes of thegraphs of FIGS. 5A and 5B in more detail. In Table 3, ‘NG’ represents acase in which the buckling index is greater than about 0.5, and ‘OK’represents a case in which the buckling index is about 0.5 or less.

TABLE 3 Thickness of second protection layer Classification 20 μm 40 μm60 μm 80 μm 100 μm 120 μm Experimental 0.40 (OK) 0.49 (OK) 0.71 (NG)0.96 (NG) 1.24 (NG) 1.52 (NG) example 1 Experimental 0.17 (OK) 0.27 (OK)0.38 (OK) 0.50 (OK) 0.64 (NG) 0.77 (NG) example 2 Experimental 0.39 (OK)0.48 (OK) 0.69 (NG) 0.91 (NG) 1.16 (NG) 1.42 (NG) example 3 Experimental0.17 (OK) 0.26 (OK) 0.37 (OK) 0.48 (OK) 0.61 (NG) 0.75 (NG) example 4Experimental 0.38 (OK) 0.46 (OK) 0.64 (NG) 0.81 (NG) 0.99 (NG) 1.19 (NG)example 5 Experimental 0.16 (OK) 0.25 (OK) 0.34 (OK) 0.44 (OK) 0.54 (NG)0.65 (NG) example 6

From Table 3, it may be seen that the buckling index of the protectionmember of Experimental example 1 is about 0.5 or less when the thicknessof the second protection layer is about 20 μm or about 40 μm. It may beseen that the buckling index of the protection member of Experimentalexample 2 is about 0.5 or less when the thickness of the secondprotection layer is about 20 μm, about 40 μm, about 60 μm, or about 80μm. In the protection members of Experimental examples 1 and 2, thesecond protection layers have the same modulus of about 36 MPa at about60° C., and the first protection layers have thicknesses different fromeach other. The thickness of the first protection layer in theprotection member of Experimental example 1 is about 65 μm, and thethickness of the first protection layer in the protection member ofExperimental example 2 is about 100 μm. In the protection member ofExperimental example 2 compared to the protection member of Experimentalexample 1, it is determined that the maximum value of the thickness ofthe second protection layer increases as the thickness of the firstprotection layer increases.

In the protection members of Experimental examples 3 and 4, the secondprotection layers have the same modulus of about 124 MPa at about 60°C., and the first protection layers have thicknesses different from eachother. The thickness of the first protection layer in the protectionmember of Experimental example 3 is about 65 pm, and the thickness ofthe first protection layer in the protection member of Experimentalexample 4 is about 100 μm. Referring to Table 3, it may be seen that thebuckling index of the protection member of Experimental example 3 isabout 0.5 or less when the thickness of the second protection layer isabout 20 μm or about 40 μm. It may be seen that the buckling index ofthe protection member of Experimental example 4 is about 0.5 or lesswhen the thickness of the second protection layer is about 20 μm, about40 μm, about 60 μm, or about 80 μm. In the protection member ofExperimental example 4 compared to the protection member of Experimentalexample 3, it is determined that the maximum value of the thickness ofthe second protection layer increases as the thickness of the firstprotection layer increases.

In the protection members of Experimental examples 5 and 6, the secondprotection layers have the same modulus of about 383 MPa at about 60°C., and the first protection layers have thicknesses different from eachother. The thickness of the first protection layer in the protectionmember of Experimental example 5 is about 65 μm, and the thickness ofthe first protection layer in the protection member of Experimentalexample 6 is about 100 μm. It may be seen that the buckling index of theprotection member of Experimental example 5 is about 0.5 or less whenthe thickness of the second protection layer is about 20 μm or about 40μm. It may be seen that the buckling index of the protection member ofExperimental example 6 is about 0.5 or less when the thickness of thesecond protection layer is about 20 μm, about 40 μm, about 60 μm, orabout 80 μm. In the protection member of Experimental example 6 comparedto the protection member of Experimental example 5, it is determinedthat the maximum value of the thickness of the second protection layerincreases as the thickness of the first protection layer increases.

Equation (1) described above is derived from the graphs of FIGS. 5A and5B. Equation (1) for calculating the maximum value of the thickness ofthe second protection layer is an equation which is derived from thethickness of the first protection layer, the modulus value of the secondprotection layer at about 60° C., and the thickness of the secondprotection layer, which correspond to a case when the buckling index isabout 0.5 or less in FIGS. 5A and 5B.

In a case where the protection member of Experimental example 1satisfies that the buckling index is about 0.5 or less, it may be seenthat when the maximum value of the thickness of the second protectionlayer is calculated by using Equation (1), the maximum value is about 40μm. In a case where the protection member of Experimental example 3satisfies that the buckling index is about 0.5 or less, it may be seenthat when the maximum value of the thickness of the second protectionlayer is calculated, the maximum value is about 40 μm. In a case wherethe protection member of Experimental example 5 satisfies that thebuckling index is about 0.5 or less, it may be seen that when themaximum value of the thickness of the second protection layer iscalculated by using Equation (1), the maximum value is about 41 μm.

In the protection members of Experimental examples 1, 3, and 5, thefirst protection layers have the same thickness of about 65 μm, and thesecond protection layers have different moduli at about 60° C. Themodulus of the second protection layer at about 60° C. in the protectionmember of Experimental example 1 is about 36 MPa, the modulus of thesecond protection layer at about 60° C. in the protection member ofExperimental example 3 is about 124 MPa, and the modulus of the secondprotection layer at about 60° C. in the protection member ofExperimental example 5 is about 383 MPa.

In a case where the protection member of Experimental example 2satisfies that the buckling index is about 0.5 or less, it may be seenthat when the maximum value of the thickness of the second protectionlayer is calculated by using Equation (1), the maximum value is about 80μm. In a case where the protection member of Experimental example 4satisfies that the buckling index is about 0.5 or less, it may be seenthat when the maximum value of the thickness of the second protectionlayer is calculated by using Equation (1), the maximum value is about 86μm. In a case where the protection member of Experimental example 6satisfies that the buckling index is about 0.5 or less, it may be seenthat when the maximum value of the thickness of the second protectionlayer is calculated by using Equation (1), the maximum value is about 92μm.

In the protection members of Experimental examples 2, 4, and 6, thefirst protection layers have the same thickness of about 100 μm, and thesecond protection layers have different moduli at about 60° C. Themodulus of the second protection layer at about 60° C. in the protectionmember of Experimental example 2 is about 36 MPa, the modulus of thesecond protection layer at about 60° C. in the protection member ofExperimental example 4 is about 124 MPa, and the modulus of the secondprotection layer at about 60° C. in the protection member ofExperimental example 6 is about 383 MPa. It is determined that themaximum value of the thickness of the second protection layer increasesas the modulus of the second protection layer at about 60° C. increases.

The display device according to some embodiments may include the displaypanel and the protection member arranged above the display panel. Theprotection member may include the adhesive layer arranged above thedisplay panel, the second protection layer arranged above the adhesivelayer, and the first protection layer arranged above the secondprotection layer. The thickness of the second protection layer may beless than the thickness of the first protection layer, and the modulusof the second protection layer may be less than the modulus of the firstprotection layer. The protection member, which includes the protectionlayers having different moduli, absorbs or alleviates external impact,and thus, may prevent or reduce instances of the display panel and othercomponents arranged below the protection member from being damaged dueto the impact (e.g., from dropping or external objects). Thus, thedisplay device including the protection member according to someembodiments may exhibit the characteristics of improving the impactresistance and maintaining the reliability.

Also, the display device according to some embodiments includes theadhesive layer having the modulus less than the modulus of the secondprotection layer, and thus, may exhibit the characteristics ofmaintaining the reliability even when repeatedly folded and unfolded.

The display device according to some embodiments includes the protectionmember that includes the first and second protection layers havingdifferent moduli, and thus, the impact resistance may be improved.

Also, the display device according to some embodiments includes theprotection member that satisfies the thickness range of the secondprotection layer for preventing or reducing the occurrence of buckling,and thus, the reliability may be maintained even when repeatedly foldedand unfolded.

Although aspects of some embodiments of the present disclosure have beendescribed with reference to the embodiments, it will be understood thatvarious changes and modifications of the present disclosure may be madeby one ordinary skilled in the art or one having ordinary knowledge inthe art without departing from the spirit and technical field of thepresent disclosure as hereinafter claimed.

Hence, the technical scope of the inventive concept is to be determinedby the following claims, and their equivalents, and should not belimited by the foregoing description.

What is claimed is:
 1. A display device comprising: a display panelconfigured to be folded about at least one folding axis; and aprotection member above the display panel, wherein the protection membercomprises: an adhesive layer above the display panel; a first protectionlayer above the adhesive layer, and having a first thickness and afirst-first modulus at room temperature; and a second protection layerbetween the adhesive layer and the first protection layer, and having asecond thickness less than the first thickness and a second-firstmodulus less than the first-first modulus at room temperature, wherein aminimum value of the second thickness is 20 μm, and the maximum value ofthe second thickness satisfies Equation (1) below:T2_(max)=(0.13×T1−8.25)×In M2+0.68×T1−5.1  (1) wherein in Equation (1),T2 _(max) is the maximum value of the second thickness, T1 is the firstthickness, and M2 is a second-second modulus of the second protectionlayer at 60° C.
 2. The display device of claim 1, wherein moduli of theadhesive layer are less than moduli of the first protection layer andmoduli of the second protection layer at room temperature and 60° C. 3.The display device of claim 1, wherein the second-first modulus of thesecond protection layer is 10 MPa to 500 MPa, and the second-secondmodulus is 36 MPa to 400 MPa.
 4. The display device of claim 1, whereinthe first-first modulus of the first protection layer is 4 GPa to 5 GPa,and the first protection layer has a first-second modulus at 60° C., andthe first-second modulus is 4.2 GPa to 4.4 GPa.
 5. The display device ofclaim 1, wherein a modulus of the adhesive layer at 60° C. is 10 KPa to40 KPa , and a modulus at room temperature is 20 KPa to 50 KPa.
 6. Thedisplay device of claim 1, wherein a thickness of the adhesive layer is35 μm to 80 μm.
 7. The display device of claim 1, wherein the firstprotection layer comprises at least one of polyethylene terephthalate(PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN),polycarbonate (PC), poly methyl methacrylate (PMMA), polystyrene (PS),polyvinyl chloride (PVC), polyethersulfone (PES), polyethylene (PE),polypropylene (PP), polyamide (PA), modified-polyphenylene oxide(m-PPO), polyoxymethylene (POM), polyarylsulfones (PSU), polyphenylenesulfide (PPS), polyimide (PI), polyethylenimine (PEI), polyether etherketone (PEEK), polyamide-imide (PAI), or polyarylate (PAR).
 8. Thedisplay device of claim 1, wherein the second protection layer comprisesat least one of thermoplastic polyurethane (TPU), thermoplasticpolyester elastomer (TPEE), thermoplastic vulcanizate (TPV),thermoplastic olefin (TPO), thermoplastic starch (TPS), ethylenepropylene rubber (EPDM), or polyether block amide (PEBA).
 9. The displaydevice of claim 1, wherein the first protection layer comprisespolyethylene terephthalate, and the second protection layer comprisesthermoplastic polyurethane.
 10. The display device of claim 1, whereinthe protection member further comprises a functional layer above thefirst protection layer, and the functional layer comprises at least oneof an anti-fingerprint layer or a hard coating layer.
 11. The displaydevice of claim 10, wherein a modulus of the functional layer at roomtemperature is 2 GPa to 4 GPa.
 12. The display device of claim 10,wherein a thickness of the functional layer is 5 μm to 10 μm.
 13. Thedisplay device of claim 1, wherein the protection member furthercomprises an auxiliary adhesive layer between the first protection layerand the second protection layer, and wherein moduli of the auxiliaryadhesive layer are less than moduli of the first protection layer andmoduli of the second protection layer at room temperature and 60° C. 14.The display device of claim 13, wherein the moduli of the auxiliaryadhesive layer are greater than or equal to moduli of the adhesive layerat room temperature and 60° C.
 15. The display device of claim 1,wherein the first thickness of the first protection layer is 65 μm to100 μm.
 16. The display device of claim 1, wherein the second thicknessof the second protection layer is 20 μm to 95 μm.
 17. The display deviceof claim 1, wherein a thickness of the protection member is 200 μm to250 μm.
 18. A display device comprising: a display panel configured tobe folded about at least one folding axis; and a protection member abovethe display panel, wherein the protection member comprises: a firstprotection layer above the display panel, and having a first thicknessand a first-first modulus at room temperature; a second protection layerbelow the first protection layer, and having a second thickness lessthan the first thickness and a second-first modulus less than thefirst-first modulus at room temperature; and an adhesive layer betweenthe display panel and the second protection layer, and having athird-first modulus less than the second-first modulus at roomtemperature, wherein the minimum value of the second thickness is 20 μm,and the maximum value of the second thickness satisfies Equation (1)below:T2_(max)=(0.13×T1−8.25)×ln M2+0.68×T1−5.1  (1) wherein in Equation (1),T2 _(max) is the maximum value of the second thickness, T1 is the firstthickness, and M2 is a second-second modulus of the second protectionlayer at 60° C.
 19. The display device of claim 18, wherein thesecond-first modulus of the second protection layer is 10 MPa to 500MPa, and the second-second modulus at 60° C. is 36 MPa to 400 MPa. 20.The display device of claim 18, wherein a thickness of the protectionmember is 200 μm to 250 μm, the first thickness of the first protectionlayer is 65 μm to 100 μm, the second thickness of the second protectionlayer is 95 μm or less, and a thickness of the adhesive layer is 35 μmto 80 μm.
 21. The display device of claim 18, wherein the firstprotection layer comprises at least one of polyethylene terephthalate(PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN),polycarbonate (PC), poly methyl methacrylate (PMMA), polystyrene (PS),polyvinyl chloride (PVC), polyethersulfone (PES), polyethylene (PE),polypropylene (PP), polyamide (PA), modified-polyphenylene oxide(m-PPO), polyoxymethylene (POM), polyarylsulfones (PSU), polyphenylenesulfide (PPS), polyimide (PI), polyethylenimine (PEI), polyether etherketone (PEEK), polyamide-imide (PAI), or polyarylate (PAR), and thesecond protection layer comprises at least one of thermoplasticpolyurethane (TPU), thermoplastic polyester elastomer (TPEE),thermoplastic vulcanizate (TPV), thermoplastic olefin (TPO),thermoplastic starch (TPS), ethylene propylene rubber (EPDM), orpolyether block amide (PEBA).